Weather, seasons and your mileage

Why winter mileage drops

US government testing found that a conventional car's mileage in city driving is about 15 percent lower at 20°F than at 77°F, and can be as much as 24 percent lower on very short trips. Hybrids can lose more, because the engine runs more often to make heat. Several things combine:

• Freezing weather, short tripsabout 15 to 24%A cold engine runs rich until it warms up, and short trips never let it get there.

0%Share of mileage lost, on a scale to 40%40%

• Warm-up time. A cold engine runs a richer mixture until it reaches operating temperature, and that takes longer in the cold.
• Thicker fluids. Engine oil and transmission fluid are stiffer when cold, which adds friction.
• Denser air. Cold air is denser, which adds a little drag.
• Winter fuel. Winter gasoline blends hold slightly less energy per gallon in many areas.
• Lower tire pressure. Pressure falls as temperatures drop. See tire pressure.
• Heaters and defrosters draw electrical power, and snow and slush add rolling resistance.

What helps in winter

1. Combine trips so the engine warms up once instead of several times.
2. Skip long idling warm-ups. Most modern engines warm faster when driven gently after a short wait. Clear all snow and ice from the windows and lights before you drive.
3. Park in a garage if you have one. A warmer start means a shorter warm-up.
4. Check tire pressure after a cold snap and top up to the placard figure.
5. Use seat heaters where fitted. They warm you faster than the cabin heater can.

Hot weather and air conditioning

In very hot weather, air conditioning is the main load. US government guidance says it can reduce a conventional car's mileage by more than 25 percent in very hot conditions, most of all on short trips. A few habits help:

• Park in the shade or use a windshield sunshade, so the cabin starts cooler.
• Open the windows for the first minute to let hot air out, then close them and use the AC.
• Use recirculation once the cabin is cool, as your car's manual describes.
• At low speeds, open windows usually cost less than AC. At highway speeds, the drag of open windows can cost as much or more.

Wind, rain and snow

A headwind acts like extra speed, and a strong one on the highway can lower mileage noticeably. Wet roads add rolling resistance, because the tires have to push water aside, and deep snow adds much more. Drive for safety first in all of these conditions. Mileage matters far less than arriving safely.

Normal dip or real problem?

A drop of 10 to 20 percent in the depths of winter is common. Signs that something else is going on include a lit check engine light, a sudden drop that doesn't come back in spring, a pull to one side, a hot wheel or a fuel smell. In those cases, have a qualified mechanic take a look. The maintenance guide lists the items that most often affect mileage.

Seasonal FAQ